Generally, a screen printing machine includes only one clamping unit on a printing cylinder thereof. When the clamping unit is located at a feeding position, it clamps a workpiece in place on the printing cylinder. Then, the printing cylinder continuously rotates and brings the workpiece to a position corresponding to a screen carrier, and the screen carrier is caused to move horizontally while the printing cylinder rotates, so that a printed pattern is formed on the workpiece that is located between the printing cylinder and the screen carrier. Thereafter, when the workpiece is moved to an unloading position, the clamping unit releases the workpiece, so that the workpiece leaves from the printing cylinder and the screen printing procedure is completed. Thereafter, the printing cylinder keeps rotating and reaches at the feeding position again. At this point, the clamping unit is in an open state and ready for clamping a next workpiece. Since the screen printing machine with one single clamping unit can complete only one printing operation when the printing cylinder rotates a full circle of 360 degrees, the printing efficiency thereof is very low.
To improve the performance of the conventional screen printing machine, an improved screen printing machine 1 with multiple clamping units as shown in FIG. 1 has been introduced into the market. Please refer to FIG. 1. The improved screen printing machine 1 includes a printing cylinder 10, a screen carrier 11, a first clamping unit 12, a second clamping unit 13, a first servomotor 14, a second servomotor 15, and a controller 16. The first servomotor 14 and the second servomotor 15 are driven by the controller 16 to synchronously move the screen carrier 11 and the printing cylinder 10, respectively. The first clamping unit 12 and the second clamping unit 13 are located at two diametrically opposite positions on the printing cylinder 10.
When the printing starts, the first clamping unit 12 located at a feeding position is in an open state, allowing a workpiece to be moved into the first clamping unit 12. Then, the first clamping unit 1 is closed, bringing the workpiece to move circumferentially along with the rotating printing cylinder 10. Then, printing and unloading procedures are sequentially performed. When the first clamping unit 12 completes the unloading procedure, the second clamping unit 13 is located at the feeding position and ready for clamping a next workpiece. When the second clamping unit 13 completes the printing and the unloading procedures, the first clamping unit 12 is returned to the feed position again and starts a new cycle of clamping, printing and unloading procedures. That is, the improved screen printing machine 1 is able to complete printing on two workpieces when the printing cylinder 10 rotates one full circle of 360 degrees. This largely increases the printing efficiency of the screen printing machine.
However, most screen printing machines available in the market are designed for the workpiece to be printed only once. That is, the printed pattern formed on the workpiece is a two-dimensional pattern having one single layer of ink. Therefore, when the conventional screen printing machine is to be used to form a three-dimensional printed pattern including multiple layers of ink on the workpiece, the workpiece must be manually removed from an unloading unit of the screen printing machine each time one printing procedure is completed and then, be manually put in a feeding unit of the screen printing machine to repeat the screen printing until the three-dimensional printed pattern is obtained. By doing this, the time for printing is increased and it is uneasy to control the accuracy of manual printing registration. Accordingly, the printing quality and productivity are relatively low, and the complicated procedures for the three-dimensional printing also largely increase the cost of the screen printed pattern. In view that the conventional screen printing machine has a lot of disadvantage in use, it is really desirable to improve the structure of the currently available rotary screen printing machine.